Progressive learning and memory impairment occurs during the normal course of aging (1,2,3). A strong functional link has been established between the hippocampal region of the brain and learning and memory of spatial information (4,5,6). Also, age-related changes in neuroanatomical, neurochemical and neurophysiological parameters of this structure of the brain have been well documented and causally linked to the etiology of memory impairment (5,7,8). This age-dependent impairment of spatial learning is strongly correlated with a deficit in the maintenance of neuronal activity long-term potentiation (LTP) in the hippocampus (9). LTP, a form of synaptic plasticity, is postulated as a molecular substrate of spatial learning (4,10,11). The functioning of neurons and synaptic activity are heavily dependent on calcium ions (Ca.sup.2+). For example, calcium is involved in LTP (10), in the regulation of membrane excitability (12) and serves directly as a second and third messenger in neurons (13,14). Moreover, several lines of evidence from rodent species point to the hypothesis that changes in the neuronal calcium homeostasis coincide with aging of the brain in general, and may be correlated with age-related decline in cognitive functions (15,16,17). For example, hippocampal calcium channels increase their activity in aged brain (18) and the density of L-type calcium channels is increased in aged hippocampal CA1 neurons (19). The experimental evidence has led to the suggestion that changed calcium homeostasis in aged neurons may be a contributing factor to some memory deficits caused by aging (20,21,22).
High voltage activated calcium currents were enhanced in dentate granule neurons from aged rats by EGTA introduced intracellulary in the recording electrode (23). Also, reduced field Excitatory Postsynaptic Potential (fEPSP) in the hippocampal slices of old rats relative to that observed in the younger age groups was found to be correlated with impaired spatial learning in aged animals (24,25). Further evidence supporting the hypothesis that a calcium increase in aged neurons may be implicated in memory impairment comes from in vivo studies on the role of the calcium channel blocker, nimodipine. This drug reduced the number of age-related motor impairments in aged rats (26), and enhanced their recent memory (27).
The elderly are known to exhibit an increased incidence of post-operative confusional states including post-anaesthetic delirium due to aging-enhanced sensitivity to anaesthetic agents.
An inverse relationship between age and inhaled anaesthetic requirements has been observed in both humans and experimental animals (39,40). The mechanisms underlying the age dependency of inhalational anaesthetic potencies may arise from direct changes in target tissue responsiveness (41) or from pharmacokinetic changes, i.e. drug disposition, metabolism and elimination. Previous reports failed to elucidate significant age-related changes in blood solubilities of volatile agents or in their equilibration kinetics with brain tissue during anaesthesia (42,43). Ageing, however, is associated with alterations in neuronal morphology and density (44), metabolism (45) and neurotransmitter activities (46). Accordingly, the age-dependent decrease in anaesthetic requirement is probably due to increased vulnerability of specific sites in the central nervous system to anaesthetic action.
It has been reported that ageing potentiates anaesthetic-induced synaptic depression in hippocampal slices (47). In this study (47), the effects of the volatile anaesthetic, isoflurane, on dendritic field excitatory postsynaptic potentials (fEPSP) were compared in hippocampal slices taken from young mature and old Fisher 344 rats. Application of isoflurane (1% v/v) to young brain slices produced minimal effects on the recorded fEPSPs. On the contrary, the same anaesthetic concentration depressed field responses obtained from old hippocampal slices by 42+6.8% compared with baseline values. Such increased sensitivity to anaesthetic action in the old slices was consistently observed with administration of higher isoflurane concentrations. The presynaptic afferent volley was unaffected by application of low or high anaesthetic concentration, suggesting that age-induced changes in nerve fibre conduction and probably nerve ending excitability are not involved in the increased vulnerability of old synapses to anaesthetic action. It was concluded that other synaptic sites are probably involved in the mechanisms of age-dependent potentiation of anaesthetic suppression of synaptic transmission. It has been reported (48,49) that aging-induced potentiation of anaesthetic actions on synaptic transmission could be opposed by maneuvers that decreases [Ca.sup.2+ ].sub.i.
Australian Patent No. 677,613, published May 9, 1994--Charlton et al discloses a method of reducing the damaging effect of an injury to mammalian cells by treatment of the cell or mammalian tissue in vivo with a cell membrane permeant calcium buffer. The method comprises treating mammalian tissue with a damage reducing effective amount of the calcium buffer, preferably, a BAPTA derivative. The method may be used to control the concentration of Ca.sup.2+ ions in the vicinity of ion channel pores of the cells to prevent diffusion of toxic amounts of Ca.sup.2+ ions to subcellular sites located near the source of Ca.sup.2+ influx. The buffer treatment may be applied as a prophylactic or after the mammalian tissue has sustained injury. Pharmaceutical compositions comprising the calcium buffer and method of manufacture therefor are described.
A water maze test (28) for the evaluation of spatial learning of aged rats has been widely used in neurobiological studies of hippocampal function and in the characterization of cognitive deficits in the aged rats (7,25,29,30).
In vitro studies of BAPTA-AM and EGTA-AM with hippocampal slices from young and mature rats using extra cellular field recordings from the stratum radiatum of the CA1 region showed depression of the amplitude of field excitatory post synaptic potentials (fEPSP) of up to 60% in the younger slices but enhanced the fEPSP of the perfused aged slices by 30% (38).